The present invention relates to a semiconductor device, and more particularly to a semiconductor device and a data outputting method using the semiconductor device which reduce test time by using a data mask.
In a semiconductor memory device, such as DRAM, the read operation and write operation of data must be performed correctly. Thus, it is important to prevent a fail cell from occurring in a chip. However, with developments in semiconductor technology, semiconductor devices have become highly integrated. Accordingly, the number of memory cells integrated on a single chip has gradually increased with improvements in semiconductor technology. While the production process of semiconductor devices has improved, the possibility of a fail cell within the chip is still relatively high.
If testing on such a fail cell is incorrectly performed, it becomes impossible to secure the reliability of the semiconductor memory device.
When testing to secure the reliability of a semiconductor memory device, if a test is performed per a unit of one memory cell, determining whether each memory cell has passed or failed takes an extremely large amount of time when a highly-integrated semiconductor memory device is tested, which in turn results in higher cost.
Accordingly, it is apparent that it is highly desirable to reduce the amount of test time. A multi-bit test method is one method for reducing this test time.
The multi-bit test method accesses data simultaneously, and accordingly test time can be reduced. However, a multi-bit test method tests data by compressing it, resulting in disadvantages. One such disadvantage is that the multi-bit test method cannot decrease screenability, and another is that the multi-bit test method cannot properly reflect relativity due to a difference between data paths and/or power noise.
In particular, when using the multi-bit test method, the test equipment must secure the same number of pins as that of the devices, resulting in increased cost. That is, in order for a high-speed test to be performed, the test equipment must be equipped with the same number of channels as that of the devices to be tested, and enormous cost is associated with such equipment.
A method used to address such a problem is sharing an input/output channel by using a data mask.
For example, when operating in X16 mode, it is possible to test the semiconductor memory device operating in X16 even if only 8 channels are equipped. This is made possible by dividing the data output pad into an upper data output pad DQ<0:7> and a lower data output pad DQ<8:15> and masking the lower data output pad DQ<0:7> and the upper data output pad DQ<8:15> by turns.
While it is possible to reduce cost by reducing the number of channels by sharing the channels through the data mask, there is a problem in that the reliability of the test decreases.
FIG. 1 shows the operational waveform of a core portion of a conventional semiconductor device for illustrating problems in the reliability of a conventional test.
Referring to FIG. 1, a bit line pair BL, BLB is precharged at a certain level VBLP. Charge sharing occurs when an active command ACT is applied, so that the bit line pair has a difference in potential of a certain level. When the difference in potential is generated, the difference is amplified using a sense amplifier, and thus the bit line BL is boosted to the core voltage VCORE level and the inverted bit line BLB is lowered to the ground voltage VSS level.
In order to test the data using the above-mentioned channel sharing method, read commands must be applied twice. That is, when the active command ACT is applied during test mode, the first read command RD1 is applied, and then, after a prescribed period of time passes, a second read command RD2 is applied.
The difference in potential between the bit line pair BL, BLB is small at the time point when the first read command RD1 is applied, while the difference between the bit line pair BL, BLB is sufficiently large at the time point when the second read command RD2 is applied. Accordingly, a problem occurs, in that data determined to be a failure during the first read command RD1 can be determined passing during the second read command RD2. That is, the screenability of the data decreases according to the just described simple channel sharing method.